1. Field of the Invention
The present invention relates to a nonvolatile memory device and a method for manufacturing the same, and more particularly, to a nonvolatile memory device which can improve the data storage capacity without increasing the surface area of the device, and a method for manufacturing the same.
2. Description of the Related Art
Generally, non-volatile memory devices are memory apparatuses that can retain their previous data even though their power supplies are interrupted. The nonvolatile memory device has a stack type gate structure comprising a floating gate formed by interposing an oxide film for F-N tunneling on the top of a semiconductor substrate and a control gate formed by interposing an interlayer insulating film on the top of the floating gate. These non-volatile memory devices include EPROMs capable of being electrically programmed and erased through the irradiation of a UV light and EEPROMs capable of being electrically programmed and erased.
The most important characteristic of the nonvolatile memory device is the data storage characteristic by which data programmed in the memory device can be retained for a long time.
However, in the nonvolatile memory device having a stack type gate structure in accordance with the prior art, floating and control gates are formed on a substrate and then an oxide film for insulating them from other devices is formed thereon.
Here, the oxide film contains migratory ions such as Na+ and H2O inside. Accordingly, there arouses a problem that, in the proceeding of various processes such as deposition, etching, etc., migratory ions and H2O are accumulated on an adjacent floating gate to form a data leakage path. As a result, the data programmed in the floating gate is lost along the leakage path to thus deteriorate the data storage capacity.
Accordingly, as a method for overcoming the problem, recently, there have been conducted studies on a method for gathering migratory ions for preventing migratory ions contained in an oxide film of a nonvolatile memory device.
Hereinafter, the method for gathering migratory ions in accordance with the prior art will be described with reference to the accompanying drawings.
FIG. 1 is a sectional view schematically showing the structure of a nonvolatile memory device for explaining a method for gathering migratory ions in accordance with one example of the prior art.
As shown therein, in the nonvolatile memory device in accordance with the example of the prior art, a semiconductor substrate 100 is divided into an active region and an inactive region by a device isolation film 110, a floating gate 120 and a control gate 130 are formed on the active region of the semiconductor substrate 100, and an oxide film 170 for insulating these gates from a top metal wire 180 is formed thereon. Besides, a nitride film 140 surrounding the outer walls of the floating gate 120 and control gate 130 is disposed between the floating gate 120 and control gate 130 and the oxide film 170.
The nitride film 140 gathers migratory ions contained in the oxide film 170 flowing into the floating gate in a process such as deposition or etching conducted after the formation of the oxide film 170, and, at the same time, avoids H2O contained in the oxide film 170 from penetrating into the floating gate to prevent the leakage of data programmed in the floating gate. That is, the data storage capacity of the nonvolatile memory device can be improved.
However, although the nitride film 140 plays the role of gathering migratory ions and preventing the penetration of H2O, the nitride film 140 is formed under an atmosphere containing a lot of H+ ions, to thus bring about a problem that the loss of data storage capacity occurs due to H+. Further, the nitride film gets a bigger stress on films than an oxide film does, this brings about a problem of occurring a bigger loss of data storage due to the stress of the nitride film.
Subsequently, in the prior art, not a nitride film but a PSG film is used as a film for gathering migratory ions. Hereinafter, a method for gathering migratory ions using a PSG film will be described with reference to the accompanying drawings.
FIG. 2 is a sectional view schematically showing the structure of a nonvolatile memory device for explaining a method for gathering migratory ions in accordance with another example of the prior art.
As shown therein, in the nonvolatile memory device in accordance with the another example of the prior art, a floating gate 120 and a control gate 130 are formed on an active region of a semiconductor substrate 100 divided into the active region and an inactive region by a device isolation film 110, and an oxide film 170 including a PSG film 150 is disposed between films on these gates, to thus insulate the floating gate 120 and the control gate 130 from a top metal wire 180.
At this time, the PSG film 150 disposed in the oxide film 170 is high in the concentration of phosphorous. Accordingly, in a process such as deposition or etching after the formation of the oxide film 170, Na+ ions are gathered from the migratory ions contained in the oxide film 170 accumulated to the floating gate, thereby preventing the leakage of data programmed in the floating gate 120.
However, the PSG film 150 has a high gathering rate of Na+ ions of the migratory ions contained in the oxide film, but has a low gathering rate of other migratory ions. As a result, the remaining migratory ions excluding Na+ ions are penetrated into the floating gate 120 to form a data leakage path, thereby bringing out a problem that the data programmed in the floating gate 120 is lost through the data leakage path.